1. Field of the Invention
The present invention relates to a technology for exhausting heat from a developing device in an image forming apparatus.
2. Description of the Related Art
Recently, in an image forming apparatus utilizing an electrophotographic system, apparatuses that can form a color image, such as a color copying machine and a color printer, are increasing to meet a demand from the market. As the color image forming apparatus, there are a one drum type and a tandem type. In the one drum type, plural developing elements are arranged around one image carrier (photosensitive element). In the tandem type, plural imaging units are arranged in parallel.
The one drum type has an advantage in that size and cost can be easily reduced due to the one image carrier constitution. However, it is difficult to achieve speed-up in forming an image because a full color image must be formed through plural (generally four) image formations using only one image carrier. On the other hand, the tandem type tends to be increased in size and cost because of plural imaging units arranged therein. However, speed-up can be facilitated. Since a color image is recently demanded to be formed in a speed as fast as a monochrome image is formed, the tandem type apparatus gains more attention.
As a tandem type image forming apparatus, a color image forming apparatus of a tandem type including an intermediate transfer belt supported by plural supporting rollers and plural photosensitive elements arranged in parallel to be opposed to the intermediate transfer belt is disclosed in Japanese Utility Model Application Laid-Open (JP-U) No. S59-192159 and Japanese Patent Application Laid-Open (JP-A) No. H8-160839. In the image forming apparatus, a color image can be formed on a transfer member by primarily transferring respective visible images formed on the respective photosensitive elements on the intermediate transfer belt in superimposition and secondarily transferring the visible images on the intermediate transfer belt collectively on the transfer member.
FIG. 21 is a schematic of a tandem type image forming apparatus. As shown in FIG. 21, the image forming apparatus includes a paper feed table 200, a main unit 100 above the paper feed table 200, and a scanner 300 above the main unit 100. The main unit 100 includes process cartridges 18Y, 18M, 18C, and 18K, each being unitized in a cartridge including a photosensitive element 40, and at least one of devices used for image forming process, such as a photosensitive element 40, a charger, a cleaning device, and a discharger. An optical writing device 103 that forms latent images on the photosensitive elements is disposed above the process cartridges.
A device that performs main scanning of respective light beams emitted from four light sources (not shown) corresponding to the respective photosensitive elements 40Y, 40M, 40C, and 40K using a polygon mirror 211 that is a rotary polygonal mirror serving as a deflecting unit, is used as the optical writing device 103. In the optical writing device 103, light beams emitted from the respective light sources are deflected by the polygon mirror rotationally driven by a polygon motor, and are irradiated on optical writing positions on the photosensitive elements, while being repeatedly reflected by predetermined reflecting mirrors. With this configuration, cost can be suppressed compared to a case in which one deflecting unit is provided for each of the photosensitive elements 40Y, 40M, 40C, and 40K.
Toner particles serving as developer in a developing device are stirred by a stirring screw or the like to be frictionally charged. At this time, toner particles generate heat due to frictional heat among toner particles or between toner particles and the stirring screw. This phenomenon is called “self-heating” of the developer. Heat is also generated by friction between the photosensitive elements 40Y, 40M, 40C, and 40K and a cleaning blade, or by friction between the photosensitive elements 40Y, 40M, 40C, and 40K and toner particles during development is generated. The process cartridges 18Y, 18M, 18C, and 18K are formed compact so that they can be easily taken out from the image forming apparatus. Therefore, the photosensitive element, the developer, the charger, the cleaning devices are densely arranged in each cartridge.
The image forming unit in each of the process cartridges 18Y, 18M, 18C, and 18K is accommodated in a casing. Therefore, the frictional heat tends to accumulate in the process cartridges 18Y, 18M, 18C, and 18K, and temperature inside the process cartridges 18Y, 18M, 18C, and 18K can become high. When the temperature inside the process cartridges 18Y, 18M, 18C, and 18K becomes high, the toner cannot be sufficiently charged. In sufficient charge of the toner causes toner particle scattering or concentration unevenness. Under a high temperature environment, a resistance value of rubber decreases, and, for example, when a charging roller or a developing roller has a rubber layer, a charging bias or a developing bias fluctuates, which may deteriorate formation of an excellent image. In recent years, a demand for making a apparatus compact, parts inside the apparatus are arranged more densely. As a result, parts in the tandem type color image forming apparatus, therefore, heat exhaustion from the inside of the apparatus is a major issue.
Japanese Patent No. 3121220, JP-A No. 2003-208065, and JP-A No. H10-149067 describe image forming apparatuses including plural image forming units, each having a photosensitive element, a developing device, a charger, and a cleaning device, and including an exhaust unit that exhausts to the outside of the apparatus, nitrogen oxide (NOx), ozone, or the like around the respective image forming units generated in an image forming process. In the image forming apparatuses according to Japanese Patent No. 3121220, JP-A No. 2003-208065, and JP-A No. H10-149067, exhaust ducts are arranged to face the photosensitive elements and extend in a scanning direction, so that discharged substances inside the charger are sucked to the exhaust ducts by an exhaust fan. The discharged substances sucked in the exhaust ducts are exhausted to the outside by the exhaust fan. By using the exhaust unit, heated air inside the process cartridges can be sucked and exhausted to the outside so that temperature inside the process cartridges can be suppressed from rising.
JP-A No. 2004-205999 discloses an image forming apparatus in which a developing device itself has a cooling configuration as measures for heat generation in a developing unit. According to JP-A No. 2004-205999, cooling can be performed by utilizing air flow entering through an opening in a developer restricting member.
As described in JP-U. No. S59-192159 or JP-A No. H8-160839, however, in the image forming apparatus in which writings to respective photosensitive elements are performed by utilizing a single optical writing device 103 as shown in FIG. 21, the optical writing device 103 is arranged above the respective process cartridges over them. Therefore, when exhaust ducts are arranged above the process cartridges, they must be arranged between the process cartridges and the optical writing device. Accordingly, to secure a space for providing the exhaust ducts, it is necessary to move the position of the optical writing device to an upper side. Thus, when the optical writing device is moved upwardly, the image forming apparatus is increased in size in a direction of height. As a result, the apparatus becomes tall and not convenient for all users to use.
A height of the exhaust duct can be reduced, and a position of the optical writing apparatus can be set to be not so high. However, when the height of the exhaust duct is reduced, a sectional area of the exhaust duct is also reduced, which results in reduction of an amount of air sucked to the exhaust duct. As a result, since air heated inside the process cartridge cannot be exhausted efficiently, temperature inside the process cartridge cannot be reduced sufficiently. Although a rotation speed of the exhaust fan can be increased to solve this problem, this increase power consumption.
The heat exhaust duct can be arranged on a side face of the process cartridge to exhaust heated air inside the process cartridge from the side face. However, since heated air rises, the heat exhaust duct disposed on the side face is inferior in suction efficiency compared to the heat exhaust duct disposed above a heat source, which is the process cartridge. Therefore, it is difficult to sufficiently lower the temperature inside the process cartridge.
In Japanese Patent No. 3121220, JP-A No. 2003-208065, and JP-A No. H10-149067, a duct for heat exhaustion is arranged in a space above or around the image forming unit. However, if the optical writing device is arranged just above the imaging unit, a space above the imaging unit is partially closed by a frame plate on which the optical writing device is mounted, so that a space above the imaging unit is limited. In a compact color image forming apparatus adopting the tandem system, since respective imaging units are arranged to be close to one another, spaces cannot be secured around respective sides of the imaging units. When a duct for heat exhaustion is disposed in the limited small space, workability becomes poor for a general method such as a screwing work, and powder dust due to screwing (metal or resin debris) is generated or a distal end of a projecting screw abuts on an attachable and detachable imaging unit.
In JP-A No. 2004-205999, since interior of the apparatus is densely arranged and routes for heat exhaustion are reduced, it is difficult to effectively cool the imaging unit (developing device). Particularly on a back side of the apparatus, effective cooling cannot be performed because heat is more likely to accumulate on the back side.